The Shuttle Transportation System (STS) "space shuttle" is a combination of several components, the orbiter which somewhat resembles an airplane, the external tank and two solid rocket boosters. The STS is launched by a combination of the space shuttle main engines, fueled by combustion of liquid O.sub.2 and liquid H.sub.2, and solid rocket boosters, fueled by solid propellant. Fuel for the main engines is carried in the external tank. At launch, both the main engines and the solid rocket boosters are ignited, accelerating the STS to a speed of Mach 1 in 50 seconds. The solid rocket boosters consume their propellant about 2 minutes, 10 seconds, after ignition by which time the STS has reached a speed of Mach 4.5 and an altitude of 28 miles. The solid rocket booster casings separate from the vehicle and are recovered after landing in the ocean via parachute. The orbiter proceeds under power of the main engine to an altitude of about 70 miles, at which point the main engines are cut off. Twenty seconds later the external tank is separated from the orbiter, and the tank remains in space for 30-60 minutes before descending over open ocean, breaking up and burning during re-entry. The orbiter is then powered into orbit by the smaller, onboard orbital maneuvering system engines.
At the point of separation, the external tank contains about 8,000-20,000 lbs. of unused fuel. In addition, the tank includes pressurizable containers, one with a total volume of 53,518 cubic feet, the other with a total volume of 19,563 cubic feet. By comparison, the Skylab had a total volume of about 10,000 cubic feet. That these structures are discarded after being launched to near-orbital velocity is very wasteful. Various options are available for inserting the external tanks into stable orbit, where use can be made of the large pressurizable volumes they provide. These include direct insertion using the STS itself, and providing a dedicated orbital maneuvering vehicle with guidance and attitude control for the external tank. Both options are feasible. As discussed infra, the total payload of the STS launch system can be much greater than presently used because current limitations relate to the maximum allowable on the shuttle orbiter itself. However, additional payload can be carried provided means for carrying the payload elsewhere, e.g., on the external tank, were provided.
The present invention provides such means by substituting one or more of several access doors on the external tank with an access door pallet, having attachment means carrying out the original function of the replaced access door, and having, in addition, mounting means for attaching a payload internally or externally of the tank and containing means for containing the payload in case it is to be internally mounted. Externally mounted payloads could include, for example, thrusters, maneuvering devices, guidance and attitude control systems and the like. Internally mounted payloads could include, for example, measuring instruments, chemical release devices, transmitters, extensible solar collectors, internally pressurized chambers, and the like. Embodiments of the present invention are termed herein access door pallets. Access door pallets in conjunction with the external tank effectively modify the tank's function, converting it into a research vehicle or an orbiting structure in its own right.
An advantage to substituting an access door pallet for an existing access door is that such substitution can be carried out without imposing structural modifications on the tank design, or changing existing tank specifications. Furthermore, if tank specifications change, an access door pallet can be modified and constructed independently of tank construction. An access door pallet can be installed and loaded with payload at or near the launch site. The fact that the access doors are removably attached to the tank and therefore capable of substitution, makes the access doors especially attractive for post-manufacture modification of the external tank.
Modification to doors to perform a second function are known in the art. Hooks, hangars and the like are commonly affixed to doors. Aircraft doors with deployable air-inflated escape slides are disclosed in U.S. Pat. Nos. 3,771,749, 4,106,729 and 4,125,235. Such structures are combined with a door since their function is intrinsic to that of the door. Also of interest is U.S. Pat. No. 3,151,536 disclosing a "pallet" on a hinged camera back. The device combines into one structure two functions related to taking and preserving photos. None of the described devices alter the utility or function of the structure containing the door.